In an electric brake system, a master cylinder is connected via a hydraulic passage to a wheel cylinder. When a driver operates a brake operation part, an upstream hydraulic pressure is produced in this master cylinder. This upstream hydraulic pressure is shut off by a shut-off valve, so that the wheel cylinder is not directly actuated. At that time, not only an operated variable set by the brake operation part but also an additional physical variable are added to produce a downstream hydraulic pressure in a slave cylinder (hydraulic source). Accordingly, the wheel cylinder is actuated. Such a brake system is called a Brake-by-Wire (BBW) system. Then, a conventional electric brake system which implements this brake system has been disclosed (see, for example, Patent Document 1).